(1) Field of the Invention
The present invention relates to an improvement in colored, partially stabilized zirconia ceramics. More particularly, the present invention relates to colored zirconia ceramics having an excellent iridescent hue and high strength and toughness, and a process for the preparation thereof.
(2) Description of the Related Art
Recently, partially stabilized zirconia ceramics formed by incorporating a predetermined amount of a stabilizer into zirconia ceramics has attracted attention, and since the ceramics has high strength and high toughness characteristics, it is expected that the ceramics will be widely used in various fields.
Furthermore, colored zirconia ceramics formed by incorporating a colorant into this ceramics or black zirconia ceramics formed by incorporating a colorant and carbon into this ceramics have been proposed.
The latter black zirconia ceramics is disclosed in Japanese Patent Application Laid-Open Specification No. 227770/84, and it is taught that the colorant used for this ceramics is (1) at least one metal selected from the group consisting of metals of the groups 4a, 5a and 6a of the periodic table and metals of the iron groups and/or an alloy thereof, (2) at least one member selected from the group consisting of carbides, borides, silicides and mutual solid solutions of metals of the groups 4a, 5a and 6a of the periodic table, or a combination of (1) and (2). By incorporating carbon and/or graphite into this colorant, sintering is promoted, and the black hue is easily controlled.
However, this black zirconia ceramics is defective in the following points. In the case where the colorant (1) is incorporated, since the metallic component is included in zirconia ceramics, conversion to cermet is caused, and sintering of zirconia per se is hindered and inherent characteristics of zirconia cannot be exerted. For example, high strength and high toughness characteristics cannot be obtained. In the case where the colorant (2) is incorporated, vacuum sintering is adopted but control of sintering is difficult in this sintering method. Namely, it is difficult to homogenize the atmosphere throughout the interior of one sintering furnace or to maintain identical sintering conditions in respective sintering furnaces, and therefore, color unevenness is brought about among lots of the obtained ceramics and a high strength or high toughness of the same level cannot be attained.
In the above-mentioned ceramics, by incorporating carbon and/or graphite in addition to the colorant, sintering is promoted and control of the black or dark hue is facilitated.
However, since such a carbon component is included in the starting material for incorporation of the carbon component, carbon is left unreacted in the sintered body or a porous sintered body is formed, with the result that degradation of the strength is caused and a mirror surface having a deep gloss is hardly obtained.